For many purposes it is often desirable to form some approximate estimate of the area occupied by a particular deposit, and in working out the topography of any county or district (especially for agricultural purposes), it is frequently necessary to ascertain the relative extent covered by the various formations. This information, in every instance which has come under my knowledge, has been obtained by measuring on the map with a rule, and entering the results, according to the scale, as so many square miles of surface. Nothing being more uncommon in nature than a straight line of boundary, every one who has made the attempt must be aware how impossible it is to reduce with any degree of accuracy to a geometric figure such lines as those to be found on most geological maps, and having recently had occasion to make a calculation of this nature, for Devonshire, I wish to describe a very simple method which I adopted with success.